1. Field of the Invention
The present invention relates to an inverter system to obtain several kV high voltage outputs and more particularly to a multiple inverter system designed to obtain high voltage output using a plurality of unit inverters.
2. Description of the Related Art
So far, there are many needs for energy saving by variable speed drive operation of AC motors, particularly, existing high-voltage motors. In particular, a high-voltage driving system is demanded, which is applicable directly to existing high-voltage motors; e.g., 3 k system, 6 kV system in Japan and 4.2 kV system and 2.4 kV system in overseas.
A general method so far used to construct a power conversion system for obtaining high voltage is a method to connect secondary windings of a plurality of transformer in series as described in Section 3 of the technical report titled "Multiple-Power Converter and Application Technology thereof" published from The Institute of Electrical Engineers of Japan (July, 1995).
An example of a high-voltate 12-phase inverter system that is so far often used is shown in FIG. 1.
This inverter system is composed of a rectifier 110, which converts AC to DC, a DC smoothing circuit 120 comprising a reactor 121 and a capacitor 122, inverter circuits 130 and 131, which convert DC to AC of optional frequency, transformers 140 and 141 and a load 150.
This circuit is in a structure that DC output of the rectifier 110 is commonly used, a plurality of inverter circuits 130, 131 are provided for this DC voltage and the secondary side windings of the output transformers 140, 141 are connected in series so as to obtain a desired high voltage.
A control circuit is composed of a speed command unit 162, a transmitter (OSC) 163, which decides the output frequency in the inverter circuits 130, 131, a distributor (RING) 164, which distributes the signal from the transmitter 163 to semiconductor devices in the inverter circuits 130, 131, an amplifier 165, a voltage control circuit (AVR) 166, a phase shifter (PHC), which decides a gate signal phase of the rectifier 110, a voltage detecting transformer 143, which detects the output AC voltage of the output transformers 140, 141 and the voltage detected by the voltage detecting transformer 143 is input to one of the input terminals of a comparator 134 via a reverse current preventing diode 144, a command from the speed commanding unit 162 is input to the other input terminal of the comparator 145 and a deviation obtained by the comparator 145 is given to a voltage control circuit 166.
A circuit shown in FIG. 2 is in a structure to obtain a high-voltage by combining a plurality of mutually insulated inverter circuits 130, 131 by the output transformers 140, 141, and excepting these elements, other component elements which are the same as those shown in FIG. 1 are assigned with the same reference numerals used in FIG. 1 and their explanations are omitted.
This circuit is in such a structure that an inverter circuit is provided to each of the outputs from the rectifiers 110, 111 and the secondary windings of the output transformers are connected in series so as to obtain a desired high-voltage.
In the case of the structure shown in FIG. 1 and FIG. 2, the output transformers 140, 141 are required for the outputs of the inverter circuits 130, 131, respectively and therefore, an area needed for installing them becomes large. Furthermore, to make the output transformers 140, 141 to be durable for the use from low frequency, there is such a defect that their external shape becomes larger than ordinary transformers of fixed frequency.
Further, a neutral point clamped 3 level inverter shown in FIG. 3 has been developed and put in practical use in recent years. This inverter converts the AC output from an AC power source 11 into DC by a rectifier 12 and after smoothed by capacitors 13, 14, supplies AC output obtained from a 3 level inverter circuit using 3 sets of a circuit comprising self-turn-off semiconductor devices S1.about.S4 composed of, for instance, a gate turn-off thyristor (GTO) and diodes D1.about.D6 to a load motor 16. Further, P, N indicate control buses and C indicates a neutral-point potential.
A multiple level inverter as shown in FIG. 3 has an economical problem that the connection of semiconductor devices in series becomes necessary because the circuit voltage becomes equivalent to the output voltage and a size of the system becomes large because the dielectric strength becomes high.
For a conventional system in the structure as described above, there exist such problems as shown below. As techncal problems when comprising a high-voltage converter, the following matters are pointed out.
(1) If an inverter circuit is constructed without connecting semiconductor devices in series, an output transformer are required, which is not economical. PA1 (2) If an inverter circuit is constructed by connecting semiconductor devices in series, an output transformer can be eliminated but the system may not become fully reliable because it becomes necessary to select semiconductor devices that are to be connected in series and the gate control becomes complicate. PA1 (3) In the serially connected structure of semiconductor devices, the harmonic reduction is limited as a matter of course because the output side harmonic componet is decided by PWM switching frequency of semiconductor devices. PA1 (4) If even one of a lot of semiconductor devices comprising the main circuit becomes defective, the continuous operation of the system becomes impossible and and it becomes a problem in a system demanded for the continuous operation.
Further, in particular, when the high-votage output obtained by connecting the ouput sides of a plurality of unit inverters in series is supplied to an AC load, there are problems as shown below.
FIG. 4 shows an example of a definite circuit using a this type of conventional multiple inverter system. The structure shown in FIG. 4 will be described below. That is, this circuit is provided with a rectifier 2, which converts AC voltage of an AC power source A1 into DC voltae, a unit inverter, which converts DC power of the rectifier A2 into AC power, connected to the rectifier A2 in parallel with it via a smoothing capacitor A3, provided with four bridge connected semiconductor devices A5, A6, A7, A8 of, for instance, IGBT and the like and a gate controller A40 to give a firing command in the specified order to the semiconductor devices A5.about.A8 comprising the unit inverter A9.
Although not shown in FIG. 4, a pluality of the unit inverters 9 including the same smoothing capacitor A3 as the structure described above are provided, the input sides of the unit inverters A9 are connected to the rectifier A2 in parallel with it and the output sides of the unit inverters A9 are connected in series, and an AC load A10 that is, for instance, an induction motor is connected to the ouput side of the thus connected multiple inverter.
The unit inverter A9 is provided with a bypass circuit described below to protect the unit inverter A9. The bypass circuit is connected between the buses of the input side of the AC load A10 and is composed of a bypass switch A41 comprising, for instance, a thyristor, a diode bridge comprising diodes A42, A43, A44, A45 connected between the pypass switch A41 and the AC load A10, a current detector A46 to detect load current and a switch operating circuit A47 which gives an ON command to the bypass switch A41 when the current value detected by the current detector A46 exceeds a specified value.
The byass circuit is also incorporated in other unit inverters (not shown) than the unit inverter A9.
In FIG. 4, when the unit inverter A9 is in the normal state without causing a short-circuit, etc., the bypass switch A41 is kept in the OFF state and it therefore performs nothing.
However, when the semiconductor devices A5, A8 of one of a plurality of unit inverters A9 are not short-circuitted completely but short-circuitted except, for instance, the IGBT bonding wire, the current value detected by the current detector A46 exceeds a specified value and therefore, the switch operating circuit A47 operates and the pypass switch A41 is turned ON. As a result, the short-circuit current flowing to the load A10 flows in the direction of arrow.
The operation described above is in the case of an ideal circuit where there is no delay in the operation for the period when the current detector A46 detects an abnormality of the unit inverters A9 and turns the bypass switch A41 ON. Therefore, the short-circuit current flows to the AC load A10 as a result of the short-circuit of the unit inverter A9, unless the operation of the short-circuitted unit inverter A9 is once stopped, the AC load A10 can be burnt out.